A series of anti-apicophilic pentacoordinate phosphoranes (with one chelating substituent in an O-equatorial, C-apical bonding mode at pentacoordinated phosphorus atom) bearing a para-substituted aryl group (–C6H4(p-X); X = H, CF3, F, OMe) or a mesityl (2,4,6-trimethylphenyl) group were isolated using a novel bulky bidentate ligand with two C2F5groups. These phosphoranes were stable to isomerization at room temperature, and quantitatively converted into the corresponding more stable isomers (O-apical) at elevated temperatures in solution. On the basis of a kinetic study, the free energy of activation (ΔG‡) of the stereomutation of the O-equatorial mesitylphosphorane to its O-apical isomer was higher than that of the CF3 derivative by 2.6 kcal mol−1, giving rise to a further example of the steric effect of the C2F5group to freeze the isomerization of the pentacoordinate phosphorus compounds. Kinetic measurements of the isomerization of the O-equatorial ortho-unsubstituted derivatives (–C6H4(p-X)) to the corresponding O-apical isomers suggested that the O-equatorial isomers were stabilized by the π → σ*P–O interaction in the ground state.
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